The patellar tendon jerk was examined quantitatively in 8 cases of Huntington's disease and in 27 controls using surface electrodes over the quadriceps femoris and biceps femoris muscles. The purpose was to investigate whether Gordon's patellar phenomenon was caused by a lack of antagonistic innervation or by late agonistic tonic reflex activity in Huntington's disease. Specific results could indicate that the patellar tendon jerk could be used as a diagnostic aid, as is the long loop reflex of hand muscles. A late tonic stretch response was found, appearing between 160 and 260 ms after the tendon tap. This response appeared in 6 out of the 8 Huntington's cases, even when Gordon's phenomenon was not observed. Therefore, it could be assumed that the tonic quadriceps activation gives rise to the clinically observed Gordon's phenomenon.